Sound-reproducing device



June 24, 1930.

DE FoREs SOUND REPRODUCING DEVICE 5 Sheets-Sheet 1 FiiedDec. 10. 1927 Jim 612K074 lee Z/ZZ/TWZJT @513 6/5 attozneags June 24, 1930. L, DE FOREST 1,766,612

SOUND REPRODUCING DEVICE Filed Deb. 10, 1927 5 Sheets-Sheet 2 Jgne 24, 1930. DE FOREST 1,766,612

SOUND REPRODUCING DEVICE Filed Dec. 10, 1927 5 Sheets-Sheet 3 3331; AS Qua le/1 o June 24, 19 30..

L. DE FOREST scum) REPRODUCING DEVICE Filed Deb. 10. 1927 5 Sheets-Sheet 4 Ill/ll!!! II I 1/ June 24, 1.930. DE FOREST 1,766,612

SOUND REPRODUCING DEVI GE Filed Dec. 10. 1927 5 Sheets-:Sheet 5 Alo-- MA GN E 7/ C 7 lee 06/3/"667 1 1 A /{s alien wags Patented June 24, 1930 UNITED STATES PATENT OFFICE LEE DE FOREST, OF NEW YORK N. Y., ASSIGNOR, BY MESNE ASSIGNMENTS, TO GENERAL TALKING PICTURES CORPORATION, A CORPORATION OF DELAWARE SOUND-REPRODUCING DEVICE Application filed December 10, 1927. Serial No. 239,017

This invention relates broadly to novel forms of constructions in sound reproducing devices.

One of the objects of this invention is the construction of sound reproducing devices operating on the principle of valving or controlling the escape of compressed gas by means of various types of valve arrangements actuated by telephonic electric currents.

A further object of this invention is the provision of sound reproducing devices employing compressed gas under the control of electro-magnetic operated valves.

A further object of this invention is the provision of various forms of valve arrangement in sound reproducing devices for producing an arrangement for controlling the escape of compressed gas by and in accordance with the sound waves to be reproduced as represented by telephonic currents.

These and other objects as will appear from the following description are secured by means of the constructions disclosed below.

This invention resides substantially in the combination, construction, arrangement, and relative location of parts as will appear from the following disclosure.

Referring to the drawings in which the same reference numerals will be used throughout the various views whenever possible Figure 1 is a vertical cross sectional View through one form of the device;

Fig. 2 is a cross sectional view taken on the line 2-2 of Fig. 1 looking .in the direction of. the arrows;

Fig. 3 is a vertical cross sectional view with some parts broken away showing a modified form of the valving arrangement;

Fig. i is an enlarged vertical cross sectional view showing another form of valving arrangement;

ig. 5 1s a vertical cross sectional view showing a modified form of the device; I

'Fig. 6 shows still another construction employing a valving arrangement;

Fig. 7 is a side elevational view of the turret having inclined slots therein; 1

Fig. 9 shows another arrangement for mounting the moving coil and the sleeve valve;

Fig. 10 is a top plan view of the arrangement of Fig. 9;

Fig. 11 is an enlarged side elevational view showing the slotted upper part of the gland tube; A

Fig. 12 is a horizontal cross sectional view 'of the gland tube of Fig. 11;

Fig. 13 is a cross sectional view of one of the washers which surrounds the gland tube;

Fig. 14: shows the assemblage of the gland tube and washers and means for locking them together; Y

Fig. 15 discloses still another modification in which no moving coil is employed but a mass of magnetized flakes or filings are used for valving the escaped air;

Fig. 16 shows a detail of construction employed in the device of Fig. 15;

Fig. 17 is a perforated retaining ring employed in the arrangement of Fig. 15;

Figs. 18, 19, and 20.are modified forms of the device employing magnetizable metallic flakes for the valving action; and

Figs. 21 and 22 are still another arrangement employing fabric valve for controlling the escape of the compressed air.

There are Well known in the art many types of sound reproducing devices electromagnetically operated for providing what may be termed loud speaking telephones. These devices find many uses today in 'connection with radio broadcasting, public address systems, and the like. prime principles of this invention is the provision of loud speaking devices for these usesv which is adapted to produce sound waves in large volume. In order to secure this effect I have found that by employing suitable means for valving the escape of compressed gas under constant pressure ac-r tuated by and in accordance with telephonic electric currents that sound waves of considerable magnitude may be produced.

Referring to the drawings for a better understanding of the invention there is shownin'Fig. 1 a sound reproducing device operating according to these principles. The device comprises a magnetic casing 1 of any suitable configuration having the central iron core 2 around which is wound within the casing 1 a number of turns of wire 3 to provide an electro-magnet. This winding is provided with the terminal wires 19. A suitable disc of insulating material 4 is mounted around the core and on top of the winding. The core has a threaded integral projection 5 which extends through the cas ing 1 and cooperates with the nut 6 to hold it securely in place. A longitudinal cent tral bore extends entirely through the core as shown at 8 and is connected to the hollow tube 7 to which the compressed air source is connected. The top of the casing is closed by means of the metallic disc 9 made of any suitable magnetic material which is provided with a central hole, as shown, so as to leave a small air gap between it and the core 2 which extends through the hole. The ring 10 is seated in an annular recess in the disc 9 and supports thereon a brass member 11' which may have formed integral therewith a suitable horn or mouth piece or may have merely a short tubular projection to which any suitable horn may be attached. Mounted between the ring 10 and brass cap 11 is a flexible corrugated diaphragm 12 which has the central portion cut away and formed to rovide a shallow cylindrical sleeve 13. he upper end of the core 2 has a reduced threaded portion 14 on which is mounted a brass turret 15 having a plurality of fine horizontal slits 16. The cylindrical sleeve 13 is of such a diameter as to closely engage the outer cylindrical surface of the turret 15 adjacent the slits 16 so that in its normal position the slits are just closed off thereby. A moving coil 17 of light construction, preferably wound around a thin spool of paper, aluminum or the like, is s iown at 17' secured to theunder surface of the diaphragm 12. This winding which loosely surrounds the core 2 is rovided with the terminal wires 18. The diaphragm 12, as clearly appears from Fig. 2, 18 provided near .its periphery with a plurality of small holes 20. If desired this diaphragm instead of being provided with circular corrugations may be corrugated radially or in any other manner to give it suitable'fiexibility for up and down movement.

The operation of the device is as follows: Compressed air under constant pressure from any suitable source is supplied to and through tube 7 to the brass turret 15. the winding 3 is connected through its terminals 19 to a direct current source of suitable strength so as to provide a powerful electromagnetic field in the cap between the core 2 and the disc 9. The telephonic currents which are to be reproduced as sound ares are supplied to the winding 17 through the terminal wires 18. The reaction of this winding 17 on the constant magnetic field produced by thewinding 3 causes an up'and down movement of the diaphragm 12 in accordance with the telephonic currents tlowing through the winding 17. This up and down movement of the diaphragm causes the. upper edge of the sleeve 13 to move across the fine slits so as to control the escape of the compressed air therethrough in accordance with the varying telephonic currents. These sound waves produced by the escaping air are then delivered into the small ori-- fice of the horn from which they escape. The strength of the sound waves thus generated at the base of the horn depend upon the air pressure used, the cut off area in the slits controlled by the action of the sleeve, and the size of the horn employed.

In Fig. 3 the casing is shown as before at 1. and the central core is shown as before at 2 butis provided at its outer end with an enlarged cylindrical portion 2'. The metallic cover 9 is provided in the casing 1 as before to supply the air gap within which the telephonic current coil 17 which surrounds the core 2 may move. In this case the turret screws into a threaded recess in the end of the core and as before the passage 8 opens therein. In this arrangement the dia hragm 12 instead of being provided with a shallow cylindrical sleeve lies in a plane with the fine horizontal slits 1G and is caused to vibrate in front of these slits by means of the coil 17 attached thereto. This construction is provided by merely cutting a round hole in the center of the diaphragm of sufiicient diameter so that the diaphragm will just'fit down nicely over the enter 0 lindrical surface of the turret 15. Thus tie inner edge of the orifice in the diaphragm moves up and down directly in front of the fine air slits cut in the turret. At all times an inverse proportion of the.

.air escaping from the slits enters in the space below the diaphragm and within the moving coil. The annular spool is provided with a series of holes therethrough to permit the air which esca s into this SpiN'B to pass into the air cham e r below the diaphragm. A small petcock 21 is provided through which air may be allowed to escape so as to control the amount of air which can pass from the lower space into the outer air. By this arrangement a perfect air balance is secured on both sides of the diaphragm. The air which escapes below the diaphragm is in the form of sound waves. as is the air which escapes above it but it is of opposite phase therefrom and is not employed as useful sound. The operation of this device is similar to that given above.

In Fig. 4 the casing is shown at 1 having has attached thereto a piece of suitable cushioning material such, as for example, velvet or soft rubber. If velvet is employed I prefer a fabric having a long nap. The diaphragm in this case is provided with a circular concave portion to lie within the space formed by the end of the core 2 and the lower surface of the turret 15. As before the telephonic current coil 17 is mounted on the under surface of the diaphragm and surrounds the enlarged portion 2 of the core. If desired the surface of the. diaphragm adjacent the cushioning material on the turret may be provided with similar cushioning material. The turret is provided with a threaded integral stud 23 mounted within the support 2 L attached to the brass cap piece 11. The support 24 is perforated through into the neck of the horn. The diaphragm may be provided with a hole adjacent the bore 8' so that the air may escape to the space above the diaphragm, or a short tube may project from the bore and extend through the hole in the diaphragm. As before, the diaphragm may be provided with a series of holes near its periphery to provide a balanced condition of air pressure on each side thereof. The movement of the diaphragm adjacent the cushioning material controls the frictional resistance to the passage of air through the space between the diaphragmand turret. Thus as the diaphragm is moved up and down through the agency of the telephonic current coil 17- the amount of resistance offered to the How of air will vary in accordance with the strength of these telephonic currents. Thus the air flowing from the constant pressure source is more or less modulated and converted into sound waves as it passes towards the periphery of the cone and to the small neck of the horn.

The modified arrangement shown in Fig; 5 comprises a brass casing 30 on which is integrally formed a suitable horn 33. An annular recess in the wall of this casing is shown at 31 which opens to the interior of the casing as shown. The casing is internally threaded at one end to receive the closure member 34. Within the casing is mounted a two-piece magnetic member 35 35, each piece being in T-shape forma- I tion. These pieces are secured together and to the closuremember 34.- by means of the central bolt, 37. A conical cap piece is attached to the top of this unit. Within the to permit the escape of air thereannular recess in the magnetic member are mounted the direct current winding 3 and the telephonic current winding 17, each of which" comprise two portions connected together in series and provided with the terminals 18 and 19 as before. The diaphragm 12 in this case consists of a disc corrugated near its center and mounted between the two magnetic pieces 35 so that the free peripheral edge thereof is in alignment with the annular opening of the annular recess 31.

Passages 38 extending within the wall of the casing 30 connect the annular recess 31 by means of the pipes 39 to the supply tube 40 which is to be connected to the fluid pressure source. A petcock 50 is attached to the casing 30 above the closure member 34. The operation of this device will be apparent. The varying currents within the telephonic current coil 17 will cause the edge of the diaphragm 12 to vibrate across the annular opening of the recess 31 to modulate the escaping air in accordance with the varying telephonic currents. The sound waves thus produced will escape to the atmosphere through the horn 33. The portion of the air which escapes into the space below the diaphragm 12 may be permitted to pass to the atmosphere through the petcock 50.

Modified valve arrangementsfor use in the construction of Fig. 1 are shown in Figs. 7 and 8. In the arrangement of Fig. 7 the turret 15 is provided with the fine slits 16 which are in this case at an angle with the horizontal. As a modification of this arrangement the slits might be horizontal and the upper edge of the sleeve 13 could be serrated. The purpose of this arrangement is to give a more gradual cut-off of air as the cylindrical sleeve valve moves up.

and the sleeve-valve 13 are provided with fine slits, the turret with oblique slits 16 and the sleeve valve with horizontal slits 40. By this arrangement a much larger volume of air may be controlled with a given up and down'movement in the sleeve valve than is possible with the arrangements shown in Figs. 1 and 3.

Figs. 9 and 10 show another arrangement for mounting the moving coil and sleeve valve. As shown in these figures the sleeve valve consists of a cylinder 58 having about its center a ring flange 59. This flange is cemented or otherwise suitably attached to the middle of a flexible diaphragm 60, preferably of leather or rubber, which diaphragm is stretched tightly over an upwardly projecting edge of the metal ring 61.

The flexible diaphragm is clamped tightly in place and stretched by means of the upper clamping ring 62, which upper ring holds the assemblage together by means of machine screws or the like. The lower half of the cylindrical sleeve valve 58 has wound thereon the moving coil 17 which is energized by means of the telephonic currents to be reproduced. As shown in these figures the sleeve valve 58 is slotted at 63 and the flange 59 is slotted at 64. The object of these slots is to prevent the circulation of short circuiting currents which would be otherwise set up by induction from the telephonic currents traversing the coil 17. Similar slits may be advantageously cut in any of the sleeve valves already described and as shown for instance in dotted lines in Fig. 8.

In the arrangement shown in Fig. 6 an entirely different form of valve for controlling the escape of compressed air is employed. The magnetic casing is shown at 1 provided with the core 2 attached thereto, which core is surrounded by the magnetic winding 3 having the terminal leads 19. The core is provided with the central bore 8 which is connected at one. end of the tube 7 which receives the connection from the fluid pressure source. The magnetizable cover 9 is provided for the casing 1 to which is secured the brass or other non-magnetic cap member 11 provided with a horn for guiding the sound waves. The upper end of the passage 8 in the core 2 is provided with a-short piece of pipe 51 which extends beyond the end of the core provided with the longitudinal slots 53 extendin through the wall thereof. A plurality of discs 54 are mounted upon the tube and have the periphery of the central holes therein cut in wedge-shape formation as shown in Fig. 13 at 54. These discs may be made of thin light paper or silk or any other suitable material. They might even be made of felt, soft rubber, and the like. The stacked discs 54 rest at the bottom of the stack on a flexible diaphragm 12 to the bottom of which is secured the non-magnetic cylinder 17 for supporting the telephonic current winding 17 which has the terminals 18. A soft rubber disc 56 is mounted on the top of the stack and a thumb nut 55 engages the threaded end 52 of the rojecting tube 51. By this arrangement tlie pressure on the stacked discs 54 may be varied as found desirable. The details of construction of this device are more clearly shown in Figs. 11 to 14 inclusive. Inthe operation of this arrangement the varying telephonic currents flow through the coil 17 causing it to react upon the constant magnetic field produced by the winding 3 and asa result the diaphra 12 vibrates in a vertical plane. This varies the pressure on the stacked discs 54 and as a result an amount of air varying in accordance with the strength of the telephonic currents is permitted to escape into the cap member 11 from which it is directed in the form of sound waves.

Figs. 15, 16, 17, 18, 19, and 20 disclose still another form of construction employing a compressed-gas for the urpose of generating sound waves. In the orm shown in Fig. 15 the magnetic casing at 1 houses the core 2 and its winding 3. The core is provided with the passage 8 and the connecting tube 7. The type of the casing is provided with the magnetic ring 9 which as a central opening, the ed es of which are inclined to the faces thereof? As before, the cap member 11 is provided. The upper end of the core 2 is slightly reduced in diameter and externally threaded to receive the brass hollow turret 71. A series of openings 71' are provided in the walls of the turret to permit the gas passing through the passa 'e 8 to escape tothe exterior of the turret. Llamped between the core and the turret 71 is the non-magnetic cup 72, the free edges of which closely engage the under surface of the magnetic ring 9. The telephonic current coil 17 is wound around the outside of the cup 72 and has the terminal leads 18. Mounted on each side of the ring 9 are perforated rings of thin metal to provide a space within which a plurality of magnetic flakes 74 may be confined. As shown in Fig. 16 the disc 9 may be slotted as at 75 to prevent the short circuiting of the currents induced therein. In the operation of this device when the coil 3 is magnetized the metallic flakes 74 are more or less compacted together. The gas flowing through the passage 8 and out through the holes 71 into the space between the turret 71 and the cup 72 is more or less restricted in its passage through the flakes 74 depending upon their compactness. The com actness of these flakes is varied in accor ance with the telephonic currents flowing through the winding 17 so that the amount of air passing through the flakes will be proportional to the strength of the current in the winding 17. It will be a parent that this arrangement does not emp oy a moving coil as does several of the arrangements previously described. In Fig. 17 is shown one of the perforated discs 70 which serve to confine the flakes to a limited volume. In the device of Fig. 18, in which no constant magnetic field is provided, the tube rovides the gas passage and is surrounded by the winding 17 for the telephonic currents mounted upon the s 001 81 and provided with the terminal wires 18. The tube 80 opens into a casing 82 and is proi'ided with the non-magnetic partition 84. The casing has formed integrally therewith the sound wave directing member 83. The partition 84, which is smaller than the casing, is surrounded by perforated ring 86 and the space within the casing is broken up into a number of chambers by means of the perforated partition members 85. The passage to the cylinder 83 and the passage from the tube 80 into the chamber are closed off by the perforated discs 85. The space within the chamber 82 is filled with metallic flakes. In this arrangement it will be apparent that the gas passing from the pipe 80 to the born 83 will vary in amount in accordance with the resistance to its passage through the flakes, which resistance will be proportional to the strength of the telephonic currents flowing through the coil 17.

In the arrangement of Fig. 19 a permanent magnet 87 hasmounted between the poles thereof the winding 17 which carries the telephonic currents. An insulating tube 88 passes through the center of-the winding and continues on to form a sound wave directing member as shown The tube 7 connects with the tube 88 to provide a compressed gas inlet. A plurality of perforated discs 90 and 91 break up the space in the tube 88 within the confines of the coil 17 and these spaces are filled with magnetic flakes. As before, the resistance to the flow ofgas through the tube 88 is varied in accordance with the strength of the currents flowing in the coil 17.

In the arrangement of. Fig. 20 a non-magnetic casing is shown at 92 having a conical member mounted therein as shown at 95. The gas inlet is shown at 7 and is surrounded by the "telephonic current winding 17. The casing 92 is provided with a sound wave directing member 97. The coil 17 and the conical member 95 are separated so that a small space 93 is provided in which a plurality of magnetic flakes are placed. These flakes are prevented from being blown out of the device by means of the perforated ring 94. 'Theconical member 95 is held in centered position within the casing 92 by means of the supports96. As already described, the resistance to the flow of gas through the flakes 93 is proportional to the strength of the magnetic field produced by the winding 17 In each of the devices shown in Figs. 15 to 20 inclusive the perforated devices employed to break up the space containing the flakes are for the purpose of preventing the flakes from compacting together at some particular point, which action would prevent the correct operation of device. The magnetizable flakes used in these devices may be flakes or filings of iron or nickel or may even consist of rings of fine wire of iron ,or nickel. I" find that finely flaked nickel, such as is used in the Edison storage battery, operates quite efliciently. It is preferable to coat these fiakeswith a more preventing them from becoming packed too tightly.

The arrangements ofFigsI21 and 22 show a still different means for valving the escapeof compressed gas. The magnetic casing is shown at 1 containing the core 2 and its winding 3. The casing is closed by means of the magnetic disc 9 having a central opening within which the core projects. The casing 11 is attached to the disc 9. In this case the compressed air is fed through the tube 100 mounted in the wall of the cap 11 and extends downwardly therein. Attached to the inner end of the tube is a fabric chamber 102 to the lower end of which is secured the telephonic current winding 17 and its support. This winding surrounds the upper end of the core 2 out of contact therewith and lies in' the magnetic field of the winding 3. The fabric chamber is secured to the tube by a suitable clamping band 101. The upper and lower portions of the chamber on the outside of the dotted lines are cemented so that the pores are closed off to limit the escape of gas from the central portion or equatorial zone of the fabric chamber where the maximum motion and,- therefore, the maximum change of the porosity with said motion occurs. The fabric chamber in the arrangement of Fig. 21 is in the form of an oblate spheroid. This chamber may be made up of woven or plaited strands of fine wire, preferably aluminum, although it is apparent that it may be woven of any flexble thread-like material. In this arrangement the reaction of the magnetic field of the winding 17 above the magnetic field of 2 is the telephonic current coil 17. The upper end of the core 2 opens into a hollow brass chamber 107. provided vwith a pluralityof radial holes 109. This chamber 107 is surrounded by means of a porous chamber 108 and the whole structure is enclosed within the cap 11 attached to the casing 1. In this case the flexible chamber 108 consists of a porous Woven metallic fabric comprising either fine nickel or fine iron wire or tinsel strips. As shown, this magnetic fabric casing is mounted in the air gap formed in the casing 1, which air gap is energized by the winding 3. The resistance to the flow of compressed gas through the passage 8, holes 109, and fabric chamber 108 is varied as the density of the magnetic lines across the annular air gap varies in accordance with the varying telephonic currents in the winding 17 The distortion and corresponding change in porosity of the walls of the sack are proportional to the momentary strength of the telephonic current as the magnetic woven chamber is more or less drawn down into the tapering air gap. In this arrangement we have a very perfect gas valving action with minimum of mass movement and corresponding fidelity of transformation from telephonic currents into sound waves.

It is, of course, apparent that many changes in the details of construction and relative arrangement of parts in the various devices disclosed will readily occur to those skilled in the art. For instance, it is apparent that an compressed gas other than air can be use in the operation of the device, that permanent magnets may be substituted for the constant magnetic field wind ing, and that various materials, equivalents of those disclosed for purposes of illustration, can be-employed without departing from the spirit of this invention. I do not, therefore, desire to be limited to the exact disclosure iven by Way of illustration but rather to t e underlying principles of the invention as exemplified by this disclosure and pointed'out in the appended claims.

\Vhat I seek to secure by United States Letters Patent is 1. Ina sound reproducing device employing compressed air, the combination with means for producing a magnetic field of means under the'influence of said means for controlling the escape of compressed air for producing sounds.

2. In a sound reproducing device employing escaping compressed air as the source of sound waves, the combination with means for producing a constant magnetic field. means for varying the constant field and means influenced by the resultant varying field for controlling the escape of compressed air to produce sounds.

3. In a sound reproducing device employing escaping compressed air as the source of sound waves, the combination with means for producing a constant magnetic field, means for varying the constant field and means influenced by the resultant varying field for modulating the escaping compressed air in accordance with the varying resultant field to produce sounds.

4. In a sound reproducing device employing com ressed air as the transfer medium for pro ucing sound waves from varying currents, the combination with means for producing a varying magnetic field of means for varying the resistance to the flow of the compressed air in accordance with the varying magnetic field to produce sound waves.

5. In a sound reproducin device employing compressed air for t e purpose of producing sound waves, the combination with means having a given area through 'which the compressed air may escape, and

means for varying the area of said means of electromagnetic means for influencing said second means whereby the compressed air may escape in accordance with variations in the current supplied to said electromagnetic means.

6. In a sound reproducing device employing compressed air for producin sound waves, the combination with means or valving the escape of compressed air of electromagnetic means for operating said means in accordance with telephonic currents supplied thereto.

7. In a sound reproducing device employing compressed air for producin sound waves, the combination with a va ve having a stationary and a movable part for controlling the movement of compressed air and electromagnetic means operated by telephonic currents for controlling the movable art of said valve.

8. n a sound reproducing device employing compressed alr for producing sound waves, the combination with a magnetic circuit including a core having a passage therethrough of a valve in communication with the passage, and electromagnetic means connected to said valve for valving the escape .of compressed air.

9. In a device for producing sound waves from electric currents employing compressed air as the transition medium, the combination with means for producing a constant magneticfield having an air passage, means having a plurality of openings therein connected to said passage an means for varying the size of said 0 nings and means including a winding in t e magnetic field of said means for produci a constant magnetic field connected to sai .means for varying the size of the openings whereby the area of the openings vary in accordance with the strength of the current in said windin 10.1% a device for producing sound waves from electric currents employing compressed air as the transition medium, the combination with means for producing a constant magnetic field having .an air passage, means having a plurality of openings therein connected to said passage and means including a diaphragm for varying the size of said openings and means including a winding in the magnetic field of said means for producin a constant mag-' netic field connected to said means for varying the size of the openings whereby the area of the openings vary in accordance with the strength of the current in said winding.

11. In a sound reproducing device employing compressed air, the combination with means for producing a magnetic field of means including a diaphragm and a plurality of air openings adapted to be controlled by said diaphragm and an electromagnetic winding influenced by the magnetic field of said means for vibrating the diaphragm by and in accordance with telephonic currents.

12. In a compressed a1r sound reproducing device, the combination with a casing, core and winding to produce a magnetic circuit and a cover havlng means for uidin'g sound waves of a slotted turret within said cover, means surrounding the slotted turret for controlling the slots therein and electromagnetic means for vibrating said means to vary the amount of air escaping through .the slots.

13. In a compressed air sound reproducing device, the combination with a casing, core and winding to produce a magnetlc circuit and a cover having means for iding sound waves of a slotted turret within said cover, means surrounding the slotted turret for controlling the slots therein and electromagnetic means for vibrating said means to vary the amount of air escaping through the slots, said turret having a coni- 4 cal head and said casing being conical adjacent said turret to provide an annular conical passage for guiding the escaped air.

In testimony whereof I have hereunto set my hand on this 5th day of December,

- LEE DE FOREST. 

